Differential circuit compensated with self-heating effect of active device

ABSTRACT

A differential circuit with a function to compensate unevenness observed in the differential gain thereof is disclosed. The differential circuit provides a low-pass filter in one of the paired transistors not receiving the input signal in addition to another low-pass filter that provides an average of output signals as a reference level of the differential circuit. The cut-off frequency of the filter is preferably set to be equal to the transition frequency at which the self-heating effect explicitly influences the trans-conductance of the transistor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a differential circuit.

2. Related Background Arts

An optical receiver generally includes a differential circuit to convert a voltage signal with the single phase configuration into two signals with the differential configuration. The differential circuit generally shows an uneven frequency response, specifically, the differential gain thereof rises about a few decibels (dB) in low frequencies below, for instance, 100 kHz. This is because of, what is called, the self-heating of active devices typically bipolar transistors implemented within the differential circuit. The present application provides techniques to compensate this unevenness observed in the differential gain against the frequency.

SUMMARY OF THE INVENTION

One aspect of the present application relates to a differential circuit that includes an amplifying unit and a first low-pass filter. The amplifying unit has a pair of transistors to convert a mono-phase input signal received by one of the paired transistors into two output signals complementary to each other. The first low-pass filter, which is coupled with the other of the paired transistors, has a cut-off frequency smaller than a transition frequency of the other of the paired transistors. A feature of the differential circuit is that the other of the paired transistors shows a self-heating effect below the transition frequency, which enhances the differential gain of the differential circuit and causes an uneven frequency response.

The amplifying unit further includes a current source commonly connected to the paired transistors. The current source includes a source transistor. A further feature of an embodiment of the invention is that this source transistor is driven in the operational mode of the constant base current, not the constant collector current. Because the source transistor is driven by such a manner, the scattering of the current gain caused in the other of the paired transistors is cancelled by the source transistor driven in the constant base current. A still further feature of an embodiment of the invention is that the current source includes another current source for defining the constant base current. Moreover, the other current source is determined by the reference resistor and a bias voltage applied thereto. Then, the scattering of the resistance of a resistor constituting the low-pass filter coupled with the other of the paired transistors is compensated by the reference resistor. The scattering of the resistance of the resistor in the low-pass filter has tendency same with the scattering of the resistance of the reference resistor.

Another aspect of the present application relates to an optical receiver to receive an optical signal and output electrical signals complementary to each other. The optical receiver includes a trans-impedance amplifier (TIA), a differential circuit, and a feedback unit. The TIA converts a photocurrent corresponding to the received optical signal into a voltage signal. The differential circuit converts the converted voltage signal into the output output electrical signals. The differential circuit includes an amplifying unit and a compensating unit. The feedback unit generates a difference between the output electrical signals.

A feature of the optical receiver according to embodiments is that the amplifying unit includes a pair of transistors and a current source commonly connected to the paired transistors to provide a current. One of the paired transistors receives the voltage signal, while, the compensating unit includes a first low-pass filter and a second low-pass filter. The first low-pass filter is coupled with the other of the paired transistors not receiving the voltage signal. The second low-pass filter averages the difference output from the feedback unit and provides the averaged difference to the other of the paired transistors via the first low-pass filter. The first low-pass filter has a cut-off frequency greater than the cut-off frequency of the second low-pass filter but smaller than a transition frequency attributed to a gain of the other of the paired transistors.

The other of the paired transistors has the gain in frequencies less than the transition frequency greater than the gain in frequencies higher than the transition frequency. The current source includes a source transistor driven under the operational mode of the constant base current, not the constant collector current. The source transistor and the other of the paired transistors each has the current gain, a value of which scatters around respective center values and the scattering shows tendency similar to each other. The constant base current is provided from another current source including a reference resistor. The resistance of this reference resistor scatters around a center value thereof. Concurrently, the scattering of the resistance of the reference resistor has tendency similar to the scattering of the resistance of the resistor constituting the first low-pass filter.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention will be described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various figures unless otherwise specified.

FIG. 1 is a circuit diagram of a differential circuit according to the first embodiment of the invention;

FIG. 2 shows a behavior of s-parameter S₂₁ of an HBT against frequencies;

FIG. 3 shows transfer curves of the collector current against the base voltage when the self-heating is out of consideration S₁, when the channel is heated by current S2, and when the change of the temperature cannot follow the frequency of the signal S₃, respectively;

FIG. 4 compares the frequency response of the differential circuit 1 as varying the capacitance of the capacitor of the low-pass filter from 0 to 300 pF;

FIGS. 5A and 5B are functional block diagrams of optical receivers each implemented with the differential circuit shown in FIG. 1;

FIG. 6 compares the frequency response of the differential gain of the differential circuit without the low-pass filter G₁ and with the low-pass filter G₂ to compensate the gain in low frequencies;

FIG. 7 is a circuit diagram of the differential circuit including a current source modified from the aforementioned current source Is₂₃ in FIG. 1;

FIG. 8A shows the constant current source for the base current of the source transistor; FIG. 8B shows the constant current source for the base current where the pnp-transistor in FIG. 8A is replaced by a p-MOSFET; and FIG. 8C shows the still modified current source for the constant base current;

FIGS. 9A and 9B compare the frequency response of the differential gain, where the function to cancel the scattering of the current gain of the transistor is eliminated in FIG. 9A, while, the function is implemented in FIG. 9B; and

FIGS. 10A and 10B compare two cases of without and with the function to cancel the scattering of the resistance of the resistor in the low-pass filter.

DESCRIPTION OF PREFERRED EMBODIMENTS

Next, some exemplary embodiments according to the present invention will be described as referring to drawings. In the description of the drawings, numerals or symbols same or similar to each other will refer to elements same or similar to each other without overlapping explanations.

First Embodiment

FIG. 1 is a circuit diagram of a differential circuit according to the first embodiment of the invention. The differential circuit 1, which converts a signal with a mono-phase into a differential signals complementary to each other, includes an amplifying unit 2 and the compensating unit 3. Such a differential circuit 1 shown in FIG. 1 is preferably applicable to an optical receiver of the optical communication system, in particular, as a unit to convert a voltage signal with a mono-phase that reflects an optical input power into signals complementary to each other.

The amplifying unit 2 includes a pair of transistors, Q₂₁ and Q₂₂, a pair of load resistors, R₂₄ and R₂₅, and a current source Is₂₃. The paired transistors, Q₂₁ and Q₂₂, have a type of the npn-bipolar transistor, which is preferably hetero-bipolar-transistor (HBT) with the emitter commonly connected to the current source Is₂₃, the collector coupled with respective load resistors, R₂₄ and R₂₅, and the base coupled to the input and to the compensating unit 3.

The current source Is₂₃, which is a type of the constant current source, provides current to respective transistors, Q₂₁ and Q₂₂, but a sum of currents flowing in respective transistors, Q₂₁ and Q₂₂, are kept constant to be equal to the constant current defined by the current source Is₂₃. The load resistors, R₂₄ and R₂₅, are coupled between the collector of respective transistors, Q₂₁ and Q₂₂, and the positive power supply Vcc.

The frequency response of an HBT will be described by one s-parameter S₂₁ as a reference. FIG. 2 shows the behavior of the s-parameter S₂₁ of an HBT against frequencies. As shown in FIG. 2, S₂₁ shows a substantially flat response in a region from 20 MHz to 1 GHz. However, it increases as the frequency decreases below 10 MHz. That is, the gain of an HBT becomes greater in low frequencies compared to that in high frequencies. The description provided below assumes that a frequency at which the gain S₂₁ increases as the frequency decreases is called as the transition frequency.

The performance of an HBT will be further described. FIG. 3 shows behaviors of the collector current against the base voltage, where the slope of the behaviors corresponds to the trans-conductance of a bipolar transistor. A behavior S₁ corresponds to a status when a temperature thereof is T, S₂ corresponds to a temperature of T+ΔT, and S₃ corresponds to a temperature between T and T+ΔT. Neglecting the self-heating effect of an HBT, the temperature of HBT becomes constant to be T. Operating an HBT at a bias of Vop under such a condition where the device temperature is kept constant in T, the collector current is given by the behavior S₃ and the trans-conductance (∂Ic/∂Vb) becomes gm^((C)) at the operating point Vop.

Taking the self-heating effect of and HBT, that is, the channel of an HBT is heated by carriers flowing therein, which raises a temperature of the channel by LT to shift the behavior Ic-Vb from S₁ to S₂. In such a case, the trans-conductance is given by a slope of the line connecting the point on S₁ corresponding to the condition of no carrier flowing with another point on S₂ corresponding to the condition of the carrier flowing, which is denoted by gm^((LF)) in FIG. 3. The response of the channel temperature gradually becomes unable to follow the carrier flowing therein as the frequency increases, the transfer curve Ic-Vb moves to S₃ and the trans-conductance becomes the slope of the tangential line drawn from a point at which the line connecting two operating points crosses the behavior S₃, which is denoted by gm^((HF)).

A crossover frequency over which the fluctuation of the channel temperature of HBT is unable to follow the signal frequency, which can be evaluated by thermal resistance and heat capacity of an HBT, substantially becomes consistent with the transition frequency thereof. That is, the trans-conductance of an HBT is influenced by the self-heating below the transition frequency, while, it becomes substantially constant in a region over the transition frequency, which results in a characteristic that the trans-conductance in low frequencies gm^((LF)) becomes greater than the trans-conductance in high frequencies gm^((HF)). Thus, the gain of HBT becomes greater in lower frequencies.

The amplifying unit 2 in FIG. 1 shows an uneven frequency response because the circuit is comprised of HBTs whose gain shows the frequency dependence described above; specifically, the gain thereof becomes greater in low frequencies compared to that in high frequencies. Then, compensation is necessary to even the frequency response.

Referring back to FIG. 1 again, the compensating unit 3, which compensates the frequency response of the amplifying unit 2, includes the first filter 5 comprised of a resistor R₅₁ and a capacitor C₅₂, where they constitute a first low-pass filter with an output thereof SB provided to the base of one of the paired transistors Q₂₂.

The cut-off frequency fc₁ of the first filter 5, which is determined by the product of the resistance of the resistor R₅₁ and the capacitance of the capacitor C₅₂, is preferably set to be substantially equal to the transition frequency of the transistors, Q₂₁ and Q₂₂. Details how do we determines the resistance and the capacitance will be described later.

The compensating unit 3 further includes the second filter 6 put in upstream of the first filter 5. The second filter 6, which determines characteristics of the amplifying unit 2 in low frequencies, includes a resistor R₆₁ and a capacitor C₆₂. The second filter 6 also behaves as a low-pass filter with a cut-off frequency fc₂ lower than the first cut-off frequency fc₁, that is, the time constant of the second filter 6 is greater than the time constant of the first filter 5. The second filter 6 receives an output of the amplifying unit 2 via an offset compensator as a feedback unit, which is not illustrated in FIG. 1 but refer to FIG. 5A, extracts a DC component thereof and provides thus extracted DC component to the base of the transistor Q₂₂ via the first filter 5.

The compensating unit 3 further includes a buffer comprised of a collector grounded npn transistor Q₇, namely, the emitter follower between two low-pass filters, 5 and 6. The transistor Q₇ receives the reference signal whose high frequency components are cut by the second filter 6, and transfers thus cut reference signal to the amplifying unit 2. The buffer further includes a current source Is₈ to pull down the emitter level of the transistor Q₇. Because the transistor Q₇ is configured with, what is called, the emitter follower, not only the input impedance thereof becomes high so as not to influence the cut-off characteristic of the second filter 6 but also the output impedance thereof becomes small enough so as not to influence the cut-off frequency fc₁ of the first filter 5.

Next, an algorithm to determine the resistance of the resistor R₅₁ and the capacitance of the capacitor C₅₂ will be described. An extreme condition is first assumed where the capacitance C_(u) is omitted, that is, a case where the first filter 5 operates only as an attenuator without any frequency characteristic. In such a case, the amplifying unit 2 would show a response similar to that of the s-parameter S₂₁ of an HBT shown in FIG. 2. Then, the low frequency gain G_(LF) which is raised by the self-heating and the high frequency gain G_(HF) which escapes from the self-heating are observed. For an exemplary condition shown in FIG. 2, the low frequency gain G_(LF) and the high frequency gain G_(HF) are 18.6 dB and 17.75 dB, respectively.

The trans-conductance gm of an HBT in high frequencies, the resistance R of resistor R₅₁, and the current gain β (=Ic/Ib) of the transistor, Q₂₁ and Q₂₂, determine a ratio of two gains, or a difference between them in the decibel (dB) unit. That is, the difference between two gains is:

G _(HF) −G _(LF) =ΔVb ₂ /ΔVb ₁=(gm/β)×R.  (1)

Then, the capacitance C is determined such that the cut-off frequency fc₁ becomes substantially equal to the transition frequency, that is, from an equation of fc₁=1/(2π×R×C). Most transistors or most HBT show the transition frequency from 2 to 4 MHz. For instance, setting the cut-off frequency fc₁ of the first filter 5 to be 2 MHz, the capacitance of the capacitor C₅₂ becomes about 200 pF.

FIG. 4 compares the frequency response of the differential circuit 1 as varying the capacitance of the capacitor C₅₂, where the resistance of the resistor R₅₁ is set to be 400Ω. Behaviors, G₅₀ to G₃₀₀, correspond to cases where the capacitance is 50 pF, 100 pF, 200 pF and 300 pF, respectively. As shown in FIG. 4, as the capacitance becomes smaller, the reduction of the high frequency gain begins at higher frequency. For instance, when the capacitance is set to be 50 pF, the decrease of the gain by the first filter 5 compensates the increase of the gain by the self-heating of the transistor at a frequency of about 0.1 MHz. However, the first filter 5 excessively compensates the self-heating effect between the cut-off frequency fc₁ and 0.1 MHz, where the frequency response causes a large dip.

On the other hand, when the capacitance is set larger, which lowers the cut-off frequency fc₁ of the first filter 5, the frequency response of the circuit begins to decrease in a relatively lower frequency. For instance, setting the capacitance to be 300 pF, the differential gain begins to decrease by the first filter 5 at a frequency lower than 1 MHz, while, the it shows a bulge around 1 MHz by insufficient compensation. When the capacitance is set to be 200 pF, the first filter 5 adequately compensates the increase of the differential gain due to the self-heating and the frequency response becomes substantially flat.

Next, operations of the differential circuit 1 will be described as referring to FIG. 1 again. The operation in low frequencies will be first described. Receiving the input mono-phase signal by the transistor Q₂₁, the base and emitter levels rise. Because the emitters of the paired transistors, Q₂₁ and Q₂₂, are commonly connected to the current source Is₂₃, the base-emitter bias Vbe₂ of the transistor Q₂₂ decreases as the common emitter level rises. Then, the current Ib₂₂ flowing into the base of the transistor Q₂₂ is decreased. This base current Ib₂₂ also flows in the resistor R₅₁ to cause a voltage drop thereat. The reduction of the base current Ib₂₂ automatically decreases this voltage drop but raises the base level thereof. Thus, the fluctuation of the base level of one of the paired transistors Q₂₂ occurs in a phase same to the fluctuation of the base level caused in the other transistor Q₂₁. Accordingly, the differential gain of the amplifying unit 2 is decreased.

On the other hand, the capacitor C₅₂ is regarded as a short circuit in high frequencies, which makes the voltage drop caused in the resistor R₅₁ by the base current Ib₂₂ ignorable, the base level of the transistor Q₂₂ becomes substantially constant to prevent the reduction of the differential gain of the differential circuit 1. Thus, the differential circuit 1 with the compensating unit 3 can compensate the unevenness of the gain occurred in low frequencies without reducing the differential gain thereof.

Next, some variations of the differential circuit 1 will be described. FIG. 5A is a functional block diagram of an optical receiver 10A implemented with the differential circuit 1 shown in FIG. 1, and FIG. 5B is a functional block diagram of another optical receiver 10B also implemented with the differential circuit 1. The optical receiver 10A in FIG. 5A includes an inverting amplifier 11 with a feedback resistor 12, where they constitute a trans-impedance amplifier (hereafter denoted as TIA) 14, the differential amplifier 1 including the amplifying unit 2 and the compensating unit 3, the second differential amplifier 13, and the offset canceller 15A. The differential amplifier 1 receives a voltage output of TIA 14 in the base of one of the paired transistors, Q₂₁ and Q₂₂, shown in FIG. 1.

Outputs of the amplifying unit 2 are brought to the second differential amplifier 13. The outputs of the second differential amplifier 13 are not only extracted to the outside of the optical receiver 10A but brought to the offset canceller 15A. The offset canceller 15A amplifies a difference between two outputs of the second differential amplifier 13, and generates the reference to cancel this difference in the outputs, where the reference is fed back to the base of the other transistor Q₂₂ of the amplifying unit 2 via the compensating unit 3.

The optical receiver 10B shown in FIG. 5B includes a TIA 14, the differential amplifier 1 whose reference supplied to the base of the other transistor Q₂₂ is provided from a bias generator 17 via the compensating unit 3, an offset canceller 15B, and a current subtracter 16. The offset canceller 15B generates a control signal provided to the current subtracter 16 to bypass a current from a primary current attributed to the input signal. Thus, bypassing a portion of current incoming to the optical receiver 10B attributed to the input signal, which reduces a current flowing in the feedback resistor 12 of TIA 14 and an output level of TIA 14 varies, the difference of the output of TIA 14 from the reference provided from the bias generator 17 via the compensating unit 3 is cancelled.

The gain fluctuation inherently attributed to the differential circuit, which is often accompanied with low frequency characteristics of the circuit, is conventionally compensated by putting a compensating circuit, such as an equalizer comprised of resistors and capacitors, within transmission lines. However, capacitors with large capacitance are sometimes necessary to compensate the fluctuation, and unable to integrate within a circuit monolithically. An arrangement where the equalizer is placed outside of the differential circuit 1 requires complicated interconnections between the differential circuit and the equalizer. Such an arrangement often degrades the high frequency performance of the differential circuit.

On the other hand, the differential circuit 1 of an embodiment implemented in the optical receives, 10A and/or 10B, only provides the compensating unit 3 in one of inputs of the amplifying unit 2 to compensate the gain fluctuation in low frequencies. This not only simplifies the differential circuit 1 but also is realized by an integrated circuit.

The frequency response of the differential gain of the differential circuit 1 will be described as referring to FIG. 6 which shows the frequency response G₂ of the differential circuit 1 and that G₁ of a differential circuit without the compensating unit 3. In FIG. 6, the resistance and the capacitance of the first filter 5 are 400Ω and 200 pF, respectively. As shown in FIG. 6, the frequency response G₁ is substantially flat in 62.7 dB in a range from 10 MHz to 1 GHz, but clearly shows a bulge in low frequencies from 40 kHz to 4 MHz whose peak gain becomes about 64 dB. On the other hand, the differential circuit 1 with the compensating unit 3 shows a flat gain of 62.7 dB in a wide frequency region from 100 kHz to 1 GHz. Thus, the compensating unit 3 effectively reduces the differential gain in low frequencies without reducing the gain in high frequencies.

According to the differential circuit 1 of the present embodiment, the first filter 5, which is coupled with one of paired transistors Q₂₂ not receiving the input signal and having the cut-off frequency fc₁ substantially equal to the transition frequency below which the self-heating effect of a transistor explicitly occurs, raises the base level of the transistor coupled with the filter 5 in low frequencies. In such a condition of low frequencies, two transistors are operated in a common phase to suppress the differential gain. However, the first filter 5 causes no influence to the paired transistors in high frequencies. Then, the high frequency performance of the circuit is left unchanged. Thus, the gain fluctuation of the differential circuit 1 is effectively suppressed.

The buffer put between two filters, 5 and 6, isolates two functions, one of which is to generate a DC bias condition to cancel the output offset of the differential circuit 1, while, the other is to cancel the self-heating effect described above. Moreover, the buffer has the emitter follower not only to enhance the impedance against the former input, a difference between two outputs, but also to reduce the impedance for the base of the transistor Q₂₂.

Second Embodiment

In the differential circuit 1 of the first embodiment, the compensation of the differential gain in low frequencies is determined by, as shown in equation (1), the resistance R of the resistor R₅₁, the current gain β₂ of the transistor Q₂₂, and the trans-conductance gm^((H)) in high frequencies. However, the current gain β₂ has possibility to be scattered by many reasons during the processing thereof. Accordingly, the compensation for the differential gain in low frequencies also scatters. A differential circuit according to the second embodiment compensates the scattering of the current gain of transistors.

FIG. 7 is a circuit diagram of the differential circuit 1A that includes a current source 23A modified from the aforementioned current source Is₂₃ in the first embodiment. That is, although the aforementioned current source Is₂₃ is a constant current source, the second embodiment includes a transistor Q₃₂ and a current source Is₃₁ in the modified current source 23A.

The current source Is₃₁ keeps the base current Ib_(s) of the source transistor Q₃₂. The source transistor Q₃₂ is monolithically formed with the other transistors, Q₂₁ and Q₂₂, that is, the transistor Q₃₂ is formed simultaneously with those transistors, Q₂₁ and Q₂₂, by the same process. The current source Is₃₁ provides the base current Ib_(s) of the source transistor Q₃₂, which generates the collector current Ic_(s) determined by an equation of:

Ic _(s)=β_(s) ΔIb _(s).  (2)

Considering the operation of the transistor Q₂₂, the trans-conductance gm of this transistor Q₂₂ is given by:

gm=ΔIc ₂ /ΔVbe ₂,  (3)

where Vbe₂ is the base-emitter bias. Thus, the trans-conductance is proportional to the collector current Ic₂. Also, the collector current Ic₂ is proportional to the current gain β₂ of this transistor Q₂₂. Assuming that the transistors, Q₂₁, Q₂₂, and Q₃₂, are formed simultaneously, and even the absolute value thereof is scattered, the behaviors of the scattering are same to respective transistors. That is, when the current gain β₂ of the transistor Q₂₂ becomes larger, the current gain β_(s) of the source transistor Q₃₂ becomes also larger by almost same amount, or, the current gain β₂ of the transistor Q₂₂ becomes smaller, the current gain β_(s) of the source transistor Q₃₂ also becomes smaller by the same amount.

Accordingly, the compensation of the differential gain in low frequencies becomes independent of the current gain β₂ of the transistor Q₂₂. In other words, even when the transistors are formed to have a larger current gain β, which operate to decrease the compensation of the differential gain in low frequencies but to increase the collector current Ic₂₂ of the transistor Q₂₂ and that Ic_(s) of the source transistor Q₃₂ by the same amount, which increase the trans-conductance gm of the transistor Q₂₂ and the compensation of the differential gain in low frequencies also increases. Thus, the reduction of the gain compensation in low frequencies due to the increase of the current gain β₂ of the transistor Q₂₂ and the enhance of the gain compensation in low frequencies due to the increase of the current gain β_(s) of the transistor Q₃₂ cancel to each other.

A differential circuit generally accompanies with a current mirror circuit for the common current source to keep the collector current Ic_(s) of the current source constant. That is, the current source with the current mirror circuit keeps the collector current Ic_(s) of the source transistor by adjusting the base current Ib_(s) thereof. On the other hand, the current source 23A of the present embodiment includes a transistor Q₃₂ driven by the condition of the constant base current Ib_(s), which results in a condition that the collector current Ic_(s) thereof depends on the current gain β. Accordingly, even the current gain β₂ of the transistor Q₂₂ varies due to the process instability, the current gain β_(s) of the transistor Q₃₂ traces this variation, which effectively compensates the instability of the compensation of the differential gain of the amplifying unit 2 in low frequencies.

Third Embodiment

The differential amplifier 1 according to the aforementioned embodiments compensates the enhancement of the differential gain in low frequencies by setting the base bias Vb₂₂, which is derived from the base current Ib₂₂ flowing in the resistor R₅₁, in the phase same with the base bias Vb₂₁ of the other transistor Q₂₁. However, the resistor R₅₁ also scatters the resistance thereof by, for instance, the process instability, which causes the fluctuation of the compensation amount in low frequencies.

The resistor R₅₁ with greater resistance than a designed value results in an excess compensation of the differential gain in low frequencies. On the other hand, when the resistance of the resistor R₅₁ is smaller than the designed value, the amount of the compensation becomes insufficient. In both cases, the frequency response of the amplifying unit 2 causes a fluctuation. Not only the scattering of the current gain β but the scattering of the resistance is preferably corrected.

FIG. 8A is a circuit diagram of the differential circuit 1B that compensates the differential gain in low frequencies by eliminating the influence of the scattering not only the current gain β of the transistors but the resistance. The differential circuit 1B includes the current source 23B distinguishable from that of the aforementioned circuit. The current source 23B, exactly, the current source 31A to provide the base current Ib_(s) to the source transistor Q₃₂ includes a transistor 33, a reference resistor R₃₄, a reference generator 35, and an amplifier 36.

The transistor Q₃₃, which is a pnp-type bipolar transistor, operates as a current booster, that is, the transistor Q₃₃ is driven by the differential amplifier 36, which may be a general purpose operational amplifier, and this amplifier 36 receives the constant voltage from the reference generator 35 in the inverting input thereof. Specifically, two inputs of the amplifier 36 may be regarded as the virtually short-circuit, then the output of the reference generator Vref is reflected to the inverting input of the amplifier 36. The reference resistor R₃₄ is put between the non-inverting input and the power supply Vcc; accordingly, the current flowing therein is automatically determined to be (Vcc−V_(ref))/R_(ref), where R_(ref) is the resistance of the reference resistor R₃₄. This current is provided to the base of the source transistor Q₃₂ via the pnp-transistor Q₃₃ as the base current Ib_(s). Thus, the base current Ib_(s) is reversely proportional to the resistance R_(ref) of the reference resistor R₃₄.

When the reference resistor R₃₄ is formed simultaneously with the resistor R₅₁, their resistance show the scattering similar to each other. Accordingly, when the resistance of the resistor R₅₁ increases from the designed value, which results in the excess compensation of the differential gain, the resistance of the reference resistor R₃₄ also increases, which operates as reducing the base current Ib_(s) of the source transistor Q₃₂. The reduction of the base current Ib_(s) is directly reflected to the reduction of the other base current Ib₂₂; accordingly, the excess compensation due to the increases of the resistance of the resistor R₅₁ is effectively cancelled by the reduction of the base current Ib₂₂ flowing therein. The mechanism to cancel the variation of the resistance is applicable to a case where the resistance is less than the designed value. In such a case, the base current Ib₂₂ increases and the compensation for the differential gain of the amplifying unit 2 can be adequately maintained.

The pnp-transistor Q₃₃ may be replaced by a MOS transistor as shown in FIG. 8B. A MOS transistor ignores the gate current. In a case of a bipolar transistor, the base current should be considered. Specifically, the emitter current flowing in the emitter of the transistor Q₃₃ in FIG. 8B is determined by the reference voltage Vref and the resistance of the resistor R₃₄. However, the collector current outgoing from the collector, which is the base current Ib_(s) of the transistor Q₃₂, exactly becomes a current subtracted by the base current from the emitter current. When the current gain of this pnp-transistor Q₃₄ is relatively small, the contribution of the base current to the collector current must be considered.

The current source 31B shown in FIG. 8B provides, instead of the pnp-transistor, the MOSFET. As described above, the MOSFET can ignore the gate bias current, that is, the source current coming into the source is equal to the drain current outgoing from the drain, which is the base current Ib_(s) to the source transistor Q₃₂.

Also, the constant current source has an arrangement a combination of npn-bipolar transistors and pnp-bipolar transistors, as shown in FIG. 8C. The constant current source 31C in FIG. 8C includes a current mirror circuit constituted by two pnp-transistors, Q₄₁ and Q₉₂. The current mirror circuit reflects a collector current of one of transistors to the collector of the other of transistors. In an arrangement shown in FIG. 8C, the collector current of the transistor Q₄₂ is reflected to the collector current of the other transistor Q₄₁, the latter current of which becomes the base current Ib_(s) of the source transistor Q₃₂.

The former collector current outgoing from the transistor Q₄₂ is generated by the constant current source comprised of the npn-transistor Q₄₃, the reference resistor R₃₄, the differential amplifier 36 and the reference generator 35. Specifically, because two inputs of the differential amplifier 36 can be regarded as the imaginary short circuit, the reference voltage Vref provided from the reference generator 35 is reflected to the non-inverting input of the amplifier 36. Then, the current flowing in the reference resistor R₃₄ is set to be Vref/Rref, where Rref is the resistance of the reference resistor R₃₄. Assuming the current gain of the transistor Q₄₃ is large enough, that is, the base current is far smaller than the collector current and/or the emitter current, the current flowing in the reference resistor R₃₄ is directed reflected to the collector current.

The current mirror circuit reflects this collector current of the transistor Q₄₃, which is equal to the collector current of the transistor Q₄₂, to the collector current of the other transistor Q₄₁, which is the base current Ib_(s) of the source transistor. Thus, the base current Ib_(s) of the source transistor Q₃₂ depends on the resistance of the reference resistor R₃₄. The transistors, Q₃₃, Q₄₁ and Q₄₂, may be replaced by MOSFETs, or any other equivalent active devices.

FIGS. 9A and 9B compare the frequency response of the differential gain of differential amplifiers implemented with the current source according to the first embodiment and that of the second embodiment, where the former has no elements to cancel the scattering of the current gain β of the transistors, while, the latter has a function to cancel the scattering of the current gain β. In these figures, Gb^((N)), Gb^((H)), and Gb^((L)) correspond to cases when β is just designed value, enhanced value, and insufficient value, respectively.

When the current gain β₂ of the transistor Q₂₂ is the designed value, the variation of the differential gain in low frequencies can be ignorable, that is, the substantially flat response of the amplifying unit 2 can be realized. However, when the current gain β₂ is greater or smaller than the designed value, the differential gain of the amplifying unit 2 enhances or lowers about 1 dB in low frequencies below 1 MHz.

When the cancellation of the deviation of the current gain β₂ of the transistor Q₂₂ from the designed value is implemented, the variation of the differential gain in low frequencies can be suppresses to about 0.2 dB. Thus, the variation of the differential gain in low frequencies is compensated by implementing with the function to drive the current source in the constant base current mode.

FIGS. 10A and 10B compare the function of compensating the scattering of the resistance of the resistor R₅₁, where FIG. 10A shows the variation of the differential gain of the amplifying unit 2 where behaviors Gr^((N)), Gr^((H)), and Gr^((L)) correspond to cases when the resistance is the designed value, greater than that value, and smaller than that value, respectively. The resistance is just the designed value, the differential gain shows a flat response against frequencies, but the differential gain increases/decreases when the resistance deviates from the designed one by 0.5 to 0.6 dB from that in high frequencies where the self-heating is ignorable.

When the current source 31 includes the reference resistor R₃₄ to compensate the scattering of the resistance, the variation of the differential gain in low frequencies is suppressed less than 0.2 dB, as shown in FIG. 10B. Thus, by driving the source transistor Q₃₂ in the constant base current mode, moreover, the constant base current is determined by the reference resistor R₃₄, the variation of the differential gain of the amplifying unit 2 in low frequencies can be effectively suppressed even when the performances of the transistors and the resistors implemented in the differential circuit are scattered primarily due to the process instability.

In the foregoing detailed description, the circuits of the present invention have been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be further made thereto without departing from the broader spirit and scope of the present invention. For instance, the specification describes that the cut-off frequency fc₁ of the first filter 5 is set substantially equal to the transition frequency. However, the cut-off frequency fc₁ may be lower than the transition frequency. Accordingly, the present specification and figures are to be regarded as illustrative rather than restrictive. 

We claim:
 1. A differential circuit, comprising: an amplifying unit with a pair of transistors to convert a mono-phase input signal received by one of the paired transistors into two output signals complementary to each other; and a first low-pass filter coupled with the other of the paired transistors and having a cut-off frequency smaller than a transition frequency of the other of the paired transistors, wherein the other of the paired transistors shows a self-heating effect below the transition frequency.
 2. The differential circuit of claim 1, further including a second low-pass filter to generate an average of two output signals, wherein the average is provided to the other of the paired transistor via the first low-pass filter.
 3. The differential circuit of claim 2, wherein the second low-pass filter has a cut-off frequency less than the cut-off frequency of the first low-pass filter.
 4. The differential circuit of claim 2, wherein the first low-pass filter and the second low-pass filter put an emitter follower therebetween.
 5. The differential circuit of claim 1, wherein the amplifying unit further includes a current source configured with a source transistor, and wherein the source transistor is driven under an operational mode of a constant base current.
 6. The differential circuit of claim 5, wherein the source transistor and the paired transistors have respective current gains with scatterings around a center value thereof, and wherein the scatterings of respective current gains show similar tendency.
 7. The differential circuit of claim 5, wherein the current source includes another current source to generate the constant base current for the source transistor, the other current source including a reference resistor whose resistance scatters around a center value thereof, and wherein the scattering of the resistance of the reference resistor shows similar tendency to a scattering of resistance of a resistor involved in the first low-pass filter.
 8. An optical receiver to receive an optical signal and output electrical signals complementary to each other, comprising: a trans-impedance amplifier configured to convert a photocurrent corresponding to the received optical signal into a voltage signal; a differential circuit configured to convert the voltage signal into the output electrical signals, the differential circuit including an amplifying unit and a compensating unit; and a feedback unit configured to generate a difference between the output electrical signals, wherein the amplifying unit includes a pair of transistors and a current source commonly connected to the paired transistors, one of the paired transistors receiving the voltage signal, wherein the compensating unit includes a first low-pass filter and a second low-pass filter, the first low-pass filter being coupled with the other of the paired transistors not receiving the voltage signal, the second low-pass filter averaging the difference generated by the feedback unit and providing averaged difference to the other of the paired transistors via the first low-pass filter, wherein the first low-pass filter has a cut-off frequency greater than a cut-off frequency of the second low-pass filter but smaller than a transition frequency attributed to a gain of the other of the paired transistors.
 9. The optical receiver of claim 8, wherein the other of the paired transistors has the gain in frequencies below the transition frequency greater than the trans-conductance in frequencies greater than the transition frequency by a self-heating effect.
 10. The optical receiver of claim 9, wherein the current source in the amplifying unit includes a source transistor driven under an operational mode of a constant base current.
 11. The optical receiver of claim 10, wherein the source transistor and the paired transistors each has a current gain whose value scatters around respective center values thereof, and wherein the scattering of the current gain of respective transistors shows tendency similar to each other.
 12. The differential circuit of claim 10, wherein the current source includes another current source to generate the constant base current for the source transistor, the other current source including a reference resistor whose resistance scatters around a center value thereof, and wherein the scattering of the resistance of the reference resistor has tendency similar to a scattering of resistance of a resistor involved in the first low-pass filter. 